The present invention relates to a packing element. The present invention also relates to a method to produce said element, the use of said element, and a column or reactor containing a packed bed comprising a plurality of said elements, and a method to produce such packed beds.
Packing elements are known and used for mass transfer and/or heat transfer processes in columns and chemical reactors. For example, random or dumped packing elements are normally employed in gas-liquid or liquid-liquid contact towers or columns to provide mass transfer surfaces between a downwardly flowing fluid, typically a liquid stream, and an upwardly ascending fluid, typically gas or vapor stream or another liquid stream. Random packing elements may be used in a variety of chemical and treatment processes, such as, for example, rectification, stripping, fractionating, absorbing, separating, washing, extraction, or any other chemical, heat exchange, or treatment-type processes. Generally, the discrete random packing elements have a specific geometric shape and are designed to maximize performance for a given mass transfer surface area. Because the random packing elements are generally dumped or randomly packed into the column shell in an arbitrarily orientated packed bed, it is desirable for the individual random packing elements to have both high mass transfer efficiency and good hydraulic capacity when positioned in multiple rotational orientations within the packed bed. It is noted that increasing the specific surface area of the elements will generally increase the mass transfer efficiency; however, at the same time this increased area will tend to disadvantageously increase the pressure losses in the column.
The pressure losses accompanying the flow of fluids through columns packed with packing elements are caused by simultaneous kinetic and viscous energy losses. The essential factors determining the energy loss, i.e. pressure drop, in packed beds are: rate of fluid flow, viscosity and density of the fluid; closeness and orientation of the packing elements; and size shape and surface of the packing elements. The first two variables concern the fluid phases, while the last two the solid packing elements. Increasing the pressure loss in the column beyond a particular limit will result in the phenomenon of flooding, in which liquid accumulate becomes entrained in the vapor at the top of the column. Flooding is detected by sharp increases in column differential pressure, liquid hold-up, and significant decrease in separation efficiency. Therefore it is highly desirable to avoid high pressure losses and their associated flooding in packed columns.
Depending on their rotational orientation, the packing elements have a projected area that is of variable openness for the flow of any liquid and gas phases. A very open projected area in terms of the projection on a plane perpendicular to the direction of flow (i.e. up or down) allows the gas phase to flow upwards with little resistance, which contributes then to a reduced pressure drop along the length of the column. However, a very closed projected area in the direction of flow forces the gas phase to take a longer upwards flow path, which contributes then to an increased pressure drop. So the relative openness of the projected areas of the packing elements and their relative orientation to the directions of flow (e.g. upwards and downwards in a vertical column) will determine whether the pressure drop along a packed bed is too high and the column will be flooded and its efficiency reduced.
Random packing elements of the prior art exist in a variety of shapes and materials. In general, random packing elements are constructed of metal, ceramic-type material, plastics, glass, or the like. Commonly, random packing elements are cylindrical, arcuate or “saddle-shaped” or have other, non-arcuate shapes such as toroidal, and the like. One disadvantage of the random packing elements of the prior art is that often the performance of the specific element is highly dependent on its configuration and its orientation with respect to the direction of flow of fluid streams through the element within the packed bed. For example, a Pall ring is a well-known cylinder-type packing that has a plurality of slotted walls and internal tongues or projections. When viewed along its longitudinal axis, the Pall ring presents very little surface area for mass transfer, but, when viewed perpendicularly to its longitudinal axis, the element presents a very large surface area. Because of this difference, the surface areas available for vapor/liquid or liquid/liquid contact vary with the orientation of the element, which, ultimately, affects the element's performance. In addition, the large surface area in the direction perpendicular to the longitudinal axis of the Pall ring is disadvantageous in that it tends to “shield” or inhibit fluid flow through immediately adjacent rings in the downstream flow direction.
Recently developed random packings include those disclosed in U.S. Pat. No. 5,882,772 to Raschig AG or disclosed in US 2010/0230832 A1 to Koch-Glitsch. The random packing element disclosed in US '772 has specific periodic strips that are used to form a certain surface area. These packing elements are quite irregularly shaped. Thus they do not flow easily into a column or reactor when pouring or dumping a packed bed, and they readily assume a variety of orientations in the bed with varying flow properties. For example, the strips are bent only in the x-y plane which allows a highly open projected area (approximately 90% open) to be achieved when viewed in the direction of arrow II, as in FIG. 2 of US '772. The % open area of a projection of a packing element when viewed in any direction is defined as the open area of the viewed projection divided by the total area of the projection multiplied by 100%. On skilled in the art will understand that by “any direction” that it is meant that the packing element is viewed such that the entire element can be seen. For example, the element may be viewed typically in an axial or radial direction. Likewise one skilled in the art will understand that the total area then refers to the entire area of the envelope of projection of the element on the plane perpendicular to the direction of viewing. However, when viewed in the perspective view of FIG. 1 of US '772 the % open area of this embodiment is only 42%. Furthermore, when viewed in the direction of arrow III, as in FIG. 3 of US '772, it can be seen that the projected area of this same packing is nearly completely closed (approximately 0% open). Therefore depending on the orientation of this packing element in the packed bed relative to the direction of the gas flow, either the gas can flow relatively undisturbed through the element (FIG. 2 of US '772) or it must take a relatively long path to flow around the packing element (FIG. 3 of US '772). As a result, the use of these packing elements results in irregular pressure drop fields in the packed bed and a poor distribution of the phases.
US '832 A1 discloses saddle-shaped random packing elements having somewhat more spherical shapes and attempts to make the open flow volume more uniform when the element is positioned in multiple rotational orientations. This is achieved by arranging strips along a partial torus to give a fairly open projected area in these orientations. However the partial torus is not symmetrical around all possible radial axes, and some orientations have quite open projected areas and others are quite closed. For example, an analysis of the % open area of the packing element embodiment in FIGS. 14-26 of US '832 A1 indicates that it varies from a minimum of about 9% open in FIG. 16 to about 50% in FIG. 20. As a result, the use of these packing elements still results in irregular pressure drop fields in the packed bed and a poor distribution of the phases. Furthermore some of the outer rib elements may shield inner rib elements in these packing elements. This shielding effect can reduce mass transfer efficiency by reducing the element's effective surface area for mass and/or heat transfer.
Furthermore the partial torus form of US '832 A1 does not readily flow into a column or reactor during pouring of a bed, and the irregularly shaped torus may then readily assume a variety of non-equivalent orientations in the bed (e.g. with the longitudinal or radial axis oriented in the vertical direction), thus giving rise to irregular pressure drop fields and a poor distribution of phases in the bed.
In conclusion, it would be desirable to have a packing element that is relatively easy to rapidly pack in a random manner into a packed bed by pouring or dumping and that leads to more uniform and only relatively low local pressure losses and more uniform liquid distribution and therefore avoiding premature local flooding when thus randomly packed in the packed bed.